1. Technical Field
The present invention relates to a rocking chair in which a backrest tilts rearwards against a spring portion and, more particularly, to a rocking chair in which the magnitude of resistance of a spring portion to a rearward tilting of the backrest can be adjusted. Furthermore, the present invention includes a spring unit used in the rocking chair.
2. Background Art
A rocking chair includes a spring means that imparts resistance to a rearward tilting of a backrest. However, generally, a resilience adjustment means is provided for changing the magnitude (i.e., magnitude of a reaction force of the backrest acting on a body during rocking) of resistance of the spring means to the rearward tilting of the backrest. A compression coil spring is often used as a spring means. Meanwhile, the resilience adjustment device includes a stepless type using a rotary screw and a step type using a cam or lever.
A mechanism for changing the magnitude of resistance of a spring means is roughly divided into a type of changing the magnitude of initial pressing to the spring means and a type of changing the magnitude of moment acting on the spring means. The former type is disclosed in PTL 1 and PTL 2. In PTL 1 and PTL 2, a compression coil spring is supported by a movable spring mount, the movable spring mount is supported by a peripheral surface cam and an initial elastic force of the compression coil spring is changed by rotating the peripheral surface cam.
On the other hand, PTL 3 discloses a configuration in which a compression coil spring is fitted into two extendable spring mounts and pivotably coupled to one spring mount using the one spring mount as a base and the other spring mount receives the load of rocking. The compression coil spring is pivoted by moving the other spring mount by an adjustment screw having a knob.
Load is applied to a locking spring means even when a person is not leaning against a backrest. Specifically, a pretension (preload) is applied to the spring means. The reason is that the backrest is suddenly largely inclined rearward without resistance by the leaning of a person and this is dangerous when a pretension is not applied to the spring means. Further, in the case of a synchronous type chair in which a seat is tilted rearward in conjunction with the rearward tilting of the backrest, the pretension of the locking spring also serves to hold the seat so that the seat does not tilt rearward just by seating.    PTL 1: JP-A-10-179312    PTL 2: JP-A-11-169254    PTL 3: Japanese Patent Publication No. 2519167